One of the strangest – and deadliest – incidents in U.S. history occurred on this day in 1919, when millions of gallons of molasses poured into Boston’s North End, killing 21 people, injuring 150 more and laying waste to two city blocks.

The 2.3 million gallons of molasses were released when the enormous tank it was stored in ruptured with what witnesses described as a roaring sound and a rumbling of the ground. The heavy, viscous material moved surprisingly fast – at 35 miles per hour – sending a 25 feet high, 160 feet wide wave of danger at an unsuspecting public. Victims, which included animals as well as people, were crushed, drowned, swallowed up or asphyxiated. Objects fared no better: buildings were demolished, windows shattered, railcars overturned.

Emergency personnel who responded to the scene included Boston Police, Red Cross workers, the Army and Navy.

Post-disaster analyses of the probable cause of the Great Molasses Flood have focused on the tank, which may have been designed for water and insufficiently tested.

The molasses flood led to requirements in Boston for greater documentation of plans and calculations from engineers and architects – requirements that were eventually adopted throughout the U.S.

People affected by the disaster won a class action lawsuit against the company that owned the tank, United States Industrial Alcohol Company and were awarded about $7,000 per victim.